1. Field of the Invention
The present invention relates to a power supply, and particularly to safe protection of a power supply.
2. Description of the Prior Art
Almost each electronic product needs a power supply to convert power generated by an external power source (e.g. an alternating current line or a battery) into power required by a core circuit of the electronic product. A quadrature resonance (QR) power supply can reduce switching loss of a power switch of the electronic product. In various power supplies, conversion efficiency of the QR power supply is better in theory, so the QR power supply is one of the popular power supplies.
FIG. 1 is a diagram illustrating a QR power supply 8 according to the prior art. As shown in FIG. 1, a converter 10 is a flyback converter, and a QR power controller 18 can be a monolithic integrated circuit with six pins VCC, GND, GATE, CS, FB, and QRD. The pin GATE of the power controller 18 is coupled to a control terminal of a power switch 15 to control store power and release power of a primary winding PRM. A feedback circuit 20 is used for detecting a voltage of an output terminal OUT to generate and output a feedback signal VFB to the pin FB of the QR power controller 18. The pin VCC provides power to the QR power controller 18. The QR power controller 18 can detect a through the power switch 15 or the primary winding PRM through the pin CS. A VCC power supply 12 utilizes discharge of an auxiliary winding AUX to generate the power required by the QR power controller 18 on the pin VCC. Divider resistors 14 and 13 are coupled between the auxiliary winding AUX and a ground line in series, so a voltage of the pin QRD can correspond to a voltage signal VAUX of the auxiliary winding AUX.
FIG. 2 is a diagram illustrating waveforms of signals in FIG. 1, where the voltage signal VAUX representing a voltage drop of the auxiliary winding AUX and a gate signal VGATE representing a voltage of the pin GATE are listed from top to down. The QR power controller 18 detects whether the voltage signal VAUX drops across 0V through the QR detection pin QRD. After the voltage signal VAUX drops across 0V for a delay time Δt, the QR power controller 18 enables the gate signal VGATE to expect that the power switch 15 realizes valley switching. That is to say, the power switch 15 is turned on to enters an ON time TON of a next switching period when the voltage signal VAUX is close to a minimum value, where the valley switching can effectively reduce switching loss of the power switch 15.
Generally speaking, a power supply not only needs to emphasize conversion efficiency, but also needs some protection functions, such as an over voltage protection (OVP), an over current protection (OCP), an over load protection (OLP), an over temperature protection (OTP), and so on. However, how to implement the above mentioned protection functions in an integrated circuit having limited pins often requires ingenuity and experience. That is to say, how to implement the above mentioned protection functions in the integrated circuit having limited pins is very difficult.